Cold-rolled steel strip and hot-dip coated cold-rolled steel strip for use as building material and manufacturing method thereof

ABSTRACT

A cold-rolled steel strip or hot-dip coated cold-rolled steel strip for use as a fire-proof building member has composition consisting of 0.01-0.25 wt. % C, up to 1.5 wt. % Si, 0.05-2.5 wt. % Mn, up to 0.1 wt. % P, no more than 0.02 wt. % S, 0.005-0.1 wt. % Al, 0.05-1.0 wt. % Mo, optionally 0.005-0.2 wt. % one or more selected from Ti, Nb, V and W, optionally one or more of 0.05-0.6 wt. % Cu, 0.05-0.6 wt. % Ni, 0.05-3.0 wt. % Cr and 0.0003-0.003 wt. % B and the balance being essentially Fe except inevitable impurities. The cold-rolled steel strip is manufactured by hot-rolling, acid-pickling, cold-rolling and then annealing at a temperature above its recrystallization temperature but below 950° C. The hot-dip coated cold-rolled steel strip is manufactured in the same way but in-line heating a cold-rolled steel strip at a temperature above its recrystallization temperature but below 950° C. and then immersing it into a coating bath. The annealed or hot-dip coated steel strip may be further cold-rolled with such a slight duty to induce a plastic strain of 1-5%.

BACKGROUND OF THE INVENTION

1. The present invention relates to a cold-rolled steel strip and ahot-dip coated steel strip useful as building material, and is alsoconcerned with a method of manufacturing these steel strips.

2. Fire-proof coating has been applied to a surface of a building whichneeds fire-proof construction, in order to inhibit temperature-up ofsteel material during a fire or the like. A so-called “fire-proof steel”which exhibits high strength at an elevated temperature is used in thesedays, so that a building can be kept safe even when the steel materialis heated at an elevated temperature near 600° C. Use of such fire-proofsteel enables reduction or omission of fire-proof coating. Thehigh-temperature strength is generally represented by yield strength ata high-temperature.

3. Such fire-proof steel material which has been used so far as mainstructural members for a building is usually a relatively thickhot-rolled steel sheet, although a steel sheet with or without hot-dipcoating made from a cold rolled steel strip is partially used for thepurpose.

4. Construction of a building also needs steel material for secondarystructural members, roofing and walls in addition to main structuralmembers. Cold-rolled steel sheets and hot-dip coated cold-rolled steelsheets have been often used as such members. When this kind of steelmaterial is improved in fire-proof property by enhancing itshigh-temperature strength, the same advantages as those of the mainstructural members can be expected. In this sense, there is a demand forprovision of a cold-rolled steel strip or a hot-dip coated cold-rolledsteel strip excellent in fire-proof property.

5. Such a cold-rolled steel sheet is manufactured by subjecting ahot-rolled steel strip to cold-rolling, annealing, hot-dip coating, etc.The steel sheet is sometimes reformed with a heavy duty in order toshape a building member necessary for an actual use. Therefore, thesteel sheet shall have good formability as well as a properhigh-temperature strength.

SUMMARY OF THE INVENTION

6. The present invention aims at provision of steel material useful as afire-proof building member. The steel material may be provided as acold-rolled steel strip or a hot-dip coated cold-rolled steel stripexcellent in high-temperature strength and formability. The excellenthigh-temperature property is attained by controlling alloyingcomposition of the steel strip and further improved by introduction of aplastic strain to the steel strip.

7. A newly proposed steel strip useful as a fire-proof building memberessentially consists of 0.01-0.25 wt. % C, up to 1.5 wt. % Si, 0.05-2.5wt. % Mn, up to 0.1 wt. % P, up to 0.02 wt. % S, 0.005-0.1 wt. % Al,0.05-1.0 wt. % Mo and the balance being essentially Fe except inevitableimpurities. The steel strip may contain 0.005-0.2 wt. % one or more ofTi, Nb, V and W, and/or one or more of 0.05-0.6 wt. % Cu, 0.05-0.6 wt. %Ni, 0.05-3.0 wt. % Cr and 0.0003-0.003 wt. % B.

8. A cold-rolled steel strip useful as a fire-proof building member ismanufactured as follows: A slab having the specified composition isheated at 1000-1250° C., hot-rolled at 800-950° C., coiled at 400-700°C., acid-pickled, cold-rolled at a reduction ratio of 40-90% and thenannealed at a temperature above a recrystallization temperature butbelow 950° C. Either box-type annealing or continuous annealing isapplicable.

9. A hot-dip coated cold-rolled steel strip useful as a fire-proofbuilding member is manufactured as follows: A cold-rolled steel stripprocessed in the same way is in-line heated at a temperature above arecrystallization temperature but below 950° C. in a continuous hot-dipcoating installation and then immersed into a hot-dip coating bath.

10. Any of the cold-rolled steel strip or the hot-dip coated steel sheetmay be further cold-rolled with such a slight duty of 1-5% to induce aplastic strain to the steel strip, after being annealed or hot-dipcoated, respectively. Cold-rolling with a slight duty is the mosteffective in an industrial point of view, although a plastic straincould be induced into the steel strip by application of a stretchingload or leveling. The further cold-rolling advantageously enhances thehigh-temperature properties of the obtained steel strip.

BRIEF DESCRIPTION OF THE DRAWINGS

11.FIG. 1 is a graph which shows a relationship between a cold-rollingreduction ratio after annealing and properties at a room temperature and600° C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

12. The inventors have researched and examined effects of alloyingelements on a high-temperature strength necessary for a fire-proof steelmember without deterioration of formability.

13. A hot-rolled steel strip which has been designed for use as afire-proof building member is improved in high-temperature property byaddition of such alloying elements as Mo, W, Ti or Nb which aredissolved in a steel matrix or precipitated as intermetallic compounds.On the other hand, a cold-rolled steel strip or a hot-dip coatedcold-rolled steel strip is annealed at a temperature above arecrystallization point after being cold-rolled for improvement offormability, since the steel strip as cold-rolled lacks in formability.

14. Annealing after cold-rolling effectively improves a high-temperaturestrength due to addition of Mo which exhibits the same effect as that ina conventional hot-rolled steel strip designed for use as a fire-proofbuilding member. However, a high-temperature strength necessary for thepurpose is not often realized.

15. The inventors suppose the poor high-temperature strength is causedby unexpected precipitation or the like in the annealing step aftercold-rolling. That is, a cold-rolled steel strip or a hot-dip coatedcold-rolled steel strip is in metallurgical situations different from ahot-rolled steel strip, since it is processed by cold-rolling andannealing. In this sense, it is not practical to simply apply analloying design which has been proposed for a hot-rolled steel strip toa cold-rolled steel strip or a hot-dip coated cold-rolled steel stripwith the proviso that the cold-rolled steel strip or the hot-dip coatedcold-rolled steel strip is merely different in thickness from thehot-rolled steel strip.

16. Taking into consideration the metallurgical hysteresis of thecold-rolled steel strip or the hot-dip coated cold-rolled steel strip,the inventors have found addition of Mo is the most effective amongalloying designs proposed so far, and reached an advantageous alloyingdesign using dissolution and precipitation of Mo in specifiedcomposition for enhancement of high-temperature properties. Thefire-proof and high-temperature properties may be further improved bycombinative addition of such carbide-forming elements as Ti, Nb, V andW. Formability of a steel strip necessary for use as a building memberis assured by controlled heat treatment in an annealing or hot-dipcoating step.

17. The high-temperature strength is further enhanced by introduction ofa plastic strain of 1-5% to the cold-rolled steel strip or the hot-dipcoated cold-rolled steel strip. Introduction of such a slight plasticstrain enhances a yield strength of the steel strip at a hightemperature near 600° C., so as to offer steel material useful as afire-proof building member due to its excellent fire-proof properties.Such a plastic strain is applied to the steel strip in cold state butnot hot state. Practically, the plastic strain is applied to the steelstrip by cold-rolling at a small reduction ratio.

18. The proposed alloying design will be apparent from the followingexplanation.

19. C: 0.01-0.25 wt. %

20. C is an alloying element to bestow a steel with a required strength.An effect of C on the strength becomes bigger as addition of C in anamount of 0.01 wt. % or more. However, excessive addition of C above0.25 wt. % would cause deterioration of formability or weldability.

21. Si: up to 1.5 wt. %

22. Si is an alloying element effective in improvement of strength,although excessive addition of Si above 1.5 wt. % would cause hardeningof steel and poor ductility. In case of a mother sheet for hot-dipcoating, addition of Si in amount above 0.3 wt. % causes remaining ofuncoated surface parts. In this regard, Si content shall be controlledlower. Such defects as the remaining of uncoated surface parts can beinhibited by electroplating of Fe or a ferrous alloy to a steel stripand then passing the steel strip to a hot-dip coating installation, evenif the steel strip contains Si in an amount exceeding 0.3 wt. %. In thissense, a steel strip containing Si up to 1.5 wt. % may be processed inthe same way.

23. Mn: 0.05-2.5 wt. %

24. Mn is added as a deoxidizing agent to a steel in a steel making stepand is also effective for inhibition of embrittlement at a hightemperature caused by S included as an impurity. Mn effect becomesremarkable in case of addition of Mn in an amount of 0.05 wt. % or more.However, excessive addition of Mn above 2.5 wt. % would cause poorductility.

25. P: up to 0.1 wt. %

26. P is an element effective for enhancing a strength and alsoimproving corrosion resistance in combination with Cu, althoughexcessive addition of P above 0.1 wt. % would cause embrittlement.

27. S: no more than 0.02 wt. %

28. S is a harmful element included as an inevitable impurity. Less Scontent is more preferable for the purpose. Allowable S content in theproposed steel is not more than 0.02 wt. %.

29. Al: 0.005-0.1 wt. %

30. Al is added as a deoxidizing agent to a steel and also effective forstabilization of N as AlN in the steel. This effect is realized byaddition of Al in an amount of 0.005 wt. % or more. However, excessiveaddition of Al above 0.1 wt. % would cause deterioration of formabilityand external appearance.

31. Mo: 0.05-1.0 wt. %

32. An additive Mo is dissolved or precipitated as a carbide in a steelmatrix, resulting in increase of a high-temperature strength. This Moeffect becomes remarkable by addition of Mo in an amount of 0.05 wt. %or more, but saturated at 1.0 wt. %. Excessive addition of Mo above 1.0wt. % would rather cause hardening of a steel and poor ductility.

33. Ti, Nb, V, W: each 0.005-0.2 wt. %

34. These elements are optional additives which are precipitated ascarbides effective in tensile properties at room temperature as well asa high-temperature strength. The effect on such improvement is realizedby addition of Ti, Nb, V or W in an amount of 0.005 wt. % or more.However, the effect is saturated at 0.2 wt. %, and excessive addition ofTi, Nb, V or W above 0.2 wt. % would cause hardening of a steel and poorductility.

35. Cu: 0.05-0.6 wt. %

36. Cu is an optional alloying additive which effectively improvescorrosion resistance of a steel in combination with P. Such the effectis remarkable by addition of Cu in an amount of 0.05 wt. %. However,excessive addition of Cu exceeding 0.6 wt. % would rather causepromotion of high-temperature cracking during hot-rolling.

37. Ni: 0.05-0.6 wt. %

38. Ni is an optional additive effective in corrosion resistance andinhibition of high-temperature embrittlement. Such the effect isremarkable by addition of Ni in an amount of 0.05 wt. % or more.However, Ni is such an expensive element to increase a steel makingcost, and a favorable effect on such properties is hardly expectedregardless increase of Ni content even when Ni is added in an amountabove 0.6 wt. %.

39. Cr: 0.05-3.0 wt. %

40. Cr is an optional additive effective in corrosion resistance andalso increases a high-temperature strength due to precipitation ofcarbides. Such effects are remarkable by addition of Cr in an amount of0.05 wt. % or more. However, excessive addition of Cr above 3.0 wt. %would rather cause hardening of a steel and poor ductility, but notfurther improve the corrosion resistance or the high-temperaturestrength.

41. B: 0.0003-0.003 wt. %

42. B is an optional additive which effectively strengthens grainboundaries. The B effect is remarkable by addition of B in an amount of0.0003 wt. % or more, but saturated at 0.003 wt. %.

43. Steel material having the specified composition is cast to a slab bya conventional continuous casting process and then hot-rolled to apredetermined thickness.

44. In the hot-rolling step, a slab is soaked, hot-rolled and thencoiled.

45. The soaking promotes dissolution of alloying elements in the steelmatrix and renders the slab to a state capable of hot-rolling, when theslab is heated at a temperature of 1000° C. or higher. But, anexcessively higher soaking temperature above 1250° C. would cause hotembrittlement of the slab.

46. The hot-rolling is preferably finished at 800-950° C. so as toassure oversaturated dissolution of alloying elements in the steelmatrix without remaining of work-induced ferritic grains. If the finishtemperature is lower than 800° C., the alloying elements are partiallyprecipitated in the steel matrix resulting in poor high-temperaturestrength. But, a higher finish temperature above 950° C. wouldwastefully consume a thermal energy and also put a heavy duty on aheating furnace.

47. The hot-rolled steel strip is coiled at 400-700° C. The controlledcoiling temperature is effective for maintaining the dissolution ofalloying elements without growth of intermetallic compounds or the likeduring coiling. Due to such dissolution, the steel strip after annealingor in-line heating in succession to cold-rolling is improved inhigh-temperature properties.

48. Thereafter, the hot-rolled steel strip is acid-pickled beforecold-rolling.

49. The steel strip is then cold-rolled under conventional conditions.The cold-rolling is preferably performed at a reduction ratio of 40-90%in order to promote recrystallization in the following annealing orcontinuous hot-dip coating step. Such a controlled reduction ratio isalso effective for suppressing growth into coarse crystal grains whichwould put harmful influences on formability.

50. In case of manufacturing a cold-rolled steel strip without hot-dipcoating, the steel strip after cold-rolled is directly delivered to anannealing step. In the annealing step, the steel strip is heated at atemperature above its recrystallization temperature so as to releasestrains caused by the cold-rolling and to promote sufficientrecrystallization; otherwise the heat-treated steel would be hard andinsufficient of formability. On the other hand, overheating at atemperature above 950° C. causes growth into coarse crystal grains,although the steel strip can be softened. The growth into coarse crystalgrains would reduce formability and poor external appearance of theheat-treated steel strip.

51. In case of manufacturing a hot-dip coated steel strip, the steelstrip after cold-rolled is in-line heated at a temperature above itsrecrystallization temperature but below 950° C. in a continuous hot-dipcoating installation. The in-line heating is performed in a reducingatmosphere to activate a surface layer of the steel strip and to annealthe steel strip.

52. A temperature for the in-line heating is set above arecrystallization temperature: otherwise release of strains caused bycold-rolling and recrystallization would be insufficient for softeningthe steel strip. Such low-temperature heating also causes insufficientactivation of the steel strip, resulting in remaining of uncoatedsurface parts after hot-dip coating. On the contrary, overheating at atemperature above 950° C. causes growth into coarse crystal grains oroccurrence of surface defects or the like.

53. The in-line heated steel strip is then immersed into a hot-dipcoating bath in the continuous hot-dip coating installation. The hot-dipcoating bath may be Zn, Al or Zn—Al. The steel strip coated with a Zn,Al or Zn—Al plating layer is obtained in this way.

54. The cold-rolled steel strip or the hot-dip coated cold-rolled steelstrip manufactured in the way as above-mentioned may be furthersubjected to cold-rolling under such conditions to apply a plasticstrain of 1-5% to the steel strip. The introduction of the plasticstrain effectively improves a high-temperature strength of the steelstrip.

55. The effect of the plastic strain on the high-temperature strength isnewly discovered by the inventors. The high-temperature strength isincreased in response to a degree of the plastic strain which can berepresented by a reduction ratio in such a cold-rolling step with aslight duty as defined above.

56.FIG. 1 shows the effect of a cold rolling reduction ratio onmechanical properties at a room temperature and 600° C., when a steelconsisting of 0.09 wt. % C, 0.05 wt. % Si, 0.55 wt. % Mn, 0.012 wt. % P,0.006 wt. % S, 0.035 wt. % Al, 0.31 wt. % Mo, 0.07 wt. % V and thebalance being Fe except inevitable impurities was hot-rolled,cold-rolled, annealed 1 min. at 800° C. and then further cold-rolled.

57. It is noted from FIG. 1 that a yield strength at 600° C. isincreased as increase of a cold-rolling reduction ratio. Suchimprovement on the mechanical properties can be clearly recognized at areduction ratio above 1%. Although a high yield strength at 600° C. isobtained in case of a reduction ratio above 5%, an elongation at a roomtemperature is unfavorably reduced. Decrease of an elongation means poorformability of the cold-rolled steel strip or the hot-dip coatedcold-rolled steel strip. From these results, a reduction ratio duringcold-rolling after annealing is preferably controlled within a range of1-5%.

EXAMPLE Example 1

58. Each steel having composition shown in TABLE 1 was melted and castto a slab. The slab was forged and hot-rolled to a steel strip of 4.0 mmin thickness. The hot-rolled steel strip was then cold-rolled to athickness of 1.0 mm and annealed under various conditions shown in TABLE2.

59. A test peace was cut off each steel strip manufactured in this wayand offered to tensile tests at a room temperature and 600° C. Resultsare shown in TABLE 2 in combination with an annealing temperature.

60. It is noted from TABLE 2 that steel strips having specifiedcomposition and being annealed at 650-950° C. had enough ductility at aroom temperature and a higher yield strength at 600° C. in comparisonwith comparative test pieces. Consequently, it is recognized that thesteel strips in the scope of the present invention are useful asfire-proof building members excellent in high-temperature properties.TABLE 1 ALLOYING COMPOSITIONS OF STEELS USED IN EXAMPLE 1 STEELCOMPONENTS AND CONTENTS    (unit: wt. %) MARK C Si Mn P S Al Cu Ni Cr MoTi Nb V W B NOTE I 0.13 0.02 0.55 0.011 0.016 0.022 0.01 0.01 0.01 — — —— — — COMPARATIVE II 0.11 0.01 0.68 0.006 0.011 0.042 0.02 0.01 0.02 —0.155 — — — — EXAMPLES III 0.12 0.35 0.75 0.009 0.002 0.034 0.01 0.020.02 — — 0.022 — — — IV 0.15 0.02 0.45 0.008 0.005 0.040 0.02 0.01 0.010.32 — — — — — PRESENT V 0.12 0.02 0.48 0.008 0.007 0.030 0.01 0.01 0.030.31 0.050 — — — — INVENTION VI 0.10 0.01 0.54 0.010 0.004 0.047 0.500.43 0.02 0.35 — 0.030 — — — VII 0.12 0.02 1.23 0.012 0.007 0.029 0.020.01 0.02 0.52 — — 0.080 — — VIII 0.13 0.44 0.45 0.009 0.006 0.036 0.010.02 0.01 0.19 — — 0.090 — — IX 0.09 0.03 1.45 0.010 0.006 0.037 0.020.01 0.02 0.42 — — — 0.080 — X 0.08 0.06 0.77 0.012 0.009 0.031 0.010.01 0.02 0.32 — — 0.095 0.015 — XI 0.20 0.05 2.15 0.011 0.004 0.0550.01 0.01 0.55 0.30 — — — — 0.002

61. TABLE 2 EFFECTS OF ANNEALING TEMPERATURE ON TENSILE PROPERTIES OFCOLD-ROLLED STEEL STRIPS TENSILE PROPERTIES AT A ROOM TEMPERATURE YIELDANNEALING TENSILE YIELD STRENGTH EXAMPLES STEEL TEMP. STRENGTH STRENGTHELONGATION (MPa) No. MARK (° C.) (MPa) (MPa) (%) AT 600° C. NOTE 1 I 820493 360 35 98 COMPARATIVE EXAMPLE 2 II 820 480 379 30 107 COMPARATIVEEXAMPLE 3 III 820 462 364 32 115 COMPARATIVE EXAMPLE 4 IV 600 576 555 11220 COMPARATIVE EXAMPLE 5 IV 850 449 341 35 164 PRESENT INVENTION 6 V820 503 414 30 176 PRESENT INVENTION 7 VI 850 481 385 31 170 PRESENTINVENTION 8 VII 600 586 553 9 259 COMPARATIVE EXAMPLE 9 VII 850 462 37135 167 PRESENT INVENTION 10 VIII 800 494 391 32 178 PRESENT INVENTION 11IX 780 481 375 34 172 PRESENT INVENTION 12 X 850 501 411 32 184 PRESENTINVENTION 13 XI 850 610 488 26 181 PRESENT INVENTION

Example 2

62. Each slab having composition shown in TABLE 3 was prepared by acontinuous casting process. The slab was soaked at 1150-1200° C.,hot-rolled to 2.3-3.0 mm in thickness at a finishing temperature of850-870° C. and coiled at 550-580° C. The hot-rolled steel strip wasthen cold-rolled to 0.8-1.2 mm in thickness.

63. One group of cold-rolled steel strips were delivered to a continuousannealing line, while the remaining of the steel strips were deliveredto a continuous hot-dip coating line. In the continuous annealing line,each steel strip was heated 40 seconds at 820° C. and then cooled downto a room temperature with or without overageing treatment. In thecontinuous hot-dip coating line, each steel strip was in-line annealed35 seconds at 800° C., cooled down to 500° C. near a temperature of acoating bath, and then immersed into a molten Zn or Zn-5% Al bath.

64. A test piece cut off each of the cold-rolled steel strips and thehot-dip coated steel strips was offered to tensile tests at a roomtemperature and 600° C. Results are shown in TABLE 4.

65. It is recognized from TABLE 4 that any of the cold-rolled steelstrips or the hot-dip coated steel strips having the specifiedcompositions and being annealed in the specified temperature range isuseful as a fire-proof building member due to its excellent ductility ata room temperature as well as a high yield strength at 600° C. comparedwith comparative steel strips. TABLE 3 ALLOYING COMPOSITIONS OF STEELSUSED IN EXAMPLE 2 STEEL COMPONENTS AND CONTENTS    (unit: wt. %) MARK CSi Mn P S Al Cu Ni Cr Mo Ti Nb V W NOTE XII 0.13 0.01 0.45 0.011 0.0050.022 0.01 0.01 0.03 — — — — — COMPARATIVE EXAMPLE XIII 0.10 0.01 0.550.010 0.008 0.028 0.01 0.01 0.40 0.38 — — — — PRESENT INVENTION XIV 0.080.50 0.87 0.012 0.005 0.035 0.49 0.45 0.02 0.41 — — 0.075 0.015 PRESENTINVENTION

66. TABLE 4 EFFECTS OF ANNEALING TEMPERATURE ON TENSILE PROPERTIES OFCOLD-ROLLED STEEL STRIPS AND HOT-DIP COATED COLD-ROLLED STEEL STRIPSTENSILE PROPERTIES AT A ROOM THICKNESS TEMPERATURE YIELD HEATING OF ATEST TENSILE YIELD STRENGTH EXAMPLE STEEL TEMP.* PIECE STRENGTH STRENGTHELONGATION (MPa) No. MARK PROCESS* (° C.) (mm) (MPa) (MPa) (%) AT 600°C. NOTE 14 XII 1 820 1.0 445 344 33 101 COMPARATIVE 15 XII 2 800 1.0 448362 31 95 EXAMPLES 16 XIII 1 820 0.8 439 374 32 164 PRESENT 17 XIII 2800 0.8 449 385 29 163 INVENTION 18 XIII 3 800 0.8 452 381 28 171 19 XIV1 820 1.0 493 392 30 176 20 XIV 2 800 1.0 488 389 28 181

Example 3

67. Each steel having composition shown in TABLE 5 was melted, cast,forged and then hot-rolled to a steel strip of 4.0 mm in thickness. Thehot-rolled steel strip was then cold-rolled to a thickness of 1.0 mm.The cold-rolled steel strip was annealed by heating 1 min. at 800° C.and cooling in an opened atmosphere. One group of the annealed steelstrips were further cold-rolled with a slight duty to induce plasticstrains.

68. A test piece was cut off each steel strip and offered to tensiletests at a room temperature and 600° C. Results are shown in TABLE 6. Itis noted from TABLE 6 that the steel strips having the specifiedcompositions and being bestowed with plastic strains of 1-5% are usefulas fire-proof members due to their excellent ductility at a roomtemperature as well as high yield strength at 600° C. compared withcomparative steel strips. TABLE 5 ALLOYING COMPOSITIONS OF STEELS USEDIN EXAMPLE 3 STEEL COMPONENTS AND CONTENTS    (unit: wt. %) MARK C Si MnP S Al Cu Ni Cr Mo Ti Nb V W B NOTE XV 0.13 0.04 0.55 0.011 0.006 0.0280.01 0.01 0.02 — — — — — — COMPARATIVE EXAMPLE XVI 0.15 0.02 0.45 0.0130.005 0.030 0.02 0.01 0.01 0.32 — — — — — PRESENT INVENTION XVII 0.050.34 0.68 0.008 0.010 0.042 0.02 0.01 0.03 — — 0.045 — — — COMPARATIVEEXAMPLE XVIII 0.11 0.02 1.45 0.010 0.008 0.031 0.01 0.01 0.02 0.42 — — —0.08 — PRESENT INVENTION XIX 0.08 0.01 0.54 0.011 0.004 0.033 0.02 0.010.03 0.35 — 0.030 — — — PRESENT INVENTION XX 0.20 0.02 0.48 0.009 0.0060.030 0.01 0.01 0.02 0.31 0.050 — — — — PRESENT INVENTION XXI 0.05 0.040.54 0.012 0.007 0.035 0.05 0.44 0.01 0.30 — — — — — PRESENT INVENTIONXXII 0.08 0.03 0.77 0.014 0.006 0.038 0.01 0.01 0.02 0.32 — — 0.095 0.01— PRESENT INVENTION XXIII 0.13 0.03 1.44 0.010 0.004 0.043 0.01 0.010.03 0.35 — — — — — PRESENT INVENTION XXIV 0.12 0.02 0.75 0.009 0.0030.033 0.02 0.02 0.02 — 0.155 0.022 — — — COMPARATIVE EXAMPLE XXV 0.050.04 2.15 0.013 0.007 0.053 0.01 0.01 0.55 0.30 — — — — 0.002 PRESENTINVENTION XXVI 0.08 0.03 1.23 0.014 0.012 0.029 0.02 0.01 0.03 0.52 — —0.080 — — PRESENT INVENTION XXVII 0.10 0.55 0.45 0.011 0.006 0.036 0.010.02 0.02 0.12 — — — 0.09 — PRESENT INVENTION

69. TABLE 6 EFFECTS OF COLD-ROLLING REDUCTION RATIO ON TENSILEPROPERTIES AT A ROOM TEMPERATURE AND 600° C. COLD-ROLLING TENSILEPROPERTIES AT A ROOM REDUCTION TEMPERATURE YIELD RATIO AFTER TENSILEYIELD STRENGTH EXAMPLE STEEL ANNEALING STRENGTH STRENGTH ELONGATION(MPa) No. MARK (%) (MPa) (MPa) (%) AT 600° C. NOTE 21 XV 2.5 413 370 3898 COMPARATIVE EXAMPLE 22 XVI 2 542 474 29 257 PRESENT INVENTION 23 XVI7 576 545 14 270 COMPARATIVE EXAMPLE 24 XVII 0 449 381 32 114COMPARATIVE EXAMPLE 25 XVII 6 463 441 13 230 COMPARATIVE EXAMPLE 26XVIII 2.5 521 465 26 239 PRESENT INVENTION 27 XIX 2 456 401 28 189PRESENT INVENTION 28 XX 1.5 622 531 23 267 PRESENT INVENTION 29 XXI 3481 425 26 202 PRESENT INVENTION 30 XXI 7 499 476 13 219 COMPARATIVEEXAMPLE 31 XXII 2.5 453 401 29 201 PRESENT INVENTION 32 XXIII 1.5 630552 23 248 PRESENT INVENTION 33 XXV 2 596 508 24 241 PRESENT INVENTION34 XXVI 2.5 563 507 28 236 PRESENT INVENTION 35 XXVII 2 511 464 29 198PRESENT INVENTION

Example 4

70. Each slab having composition shown in TABLE 7 was prepared by acontinuous casting process. The slab was soaked at 1180-1210° C.,hot-rolled to a steel strip of 2.3-3.0 mm in thickness at a finishingtemperature of 840-870° C. and then coiled at 530-580° C. The hot-rolledsteel strip was cold-rolled to a thickness of 0.6-1.0 mm.

71. One group of the cold-rolled steel strips manufactured in this waywere delivered to a continuous annealing line, while the remaining groupof the steel strips were delivered to a hot-dip coating line. In theannealing line, each steel strip was heated 40 sec. at 800° C. and thencooled down to a room temperature with or without overageing treatment.In the hot-dip coating line, each steel strip was in-line heated 35 sec.at 800° C., cooled down to 500° C. near a temperature of a coating bathand then immersed into the coating bath. A molten Zn or Zn-5% Al poolwas used as the coating bath.

72. Each steel strip was cold-rolled with a slight duty after annealingor hot-dip coating, so as to induce a plastic strain.

73. A test piece was cut off each of the cold-rolled steel strips andthe hot-dip coated steel strips and offered to tensile tests at a roomtemperature and 600° C. Results are shown in TABLE 8. It is noted fromTABLE 8 that the steel strips having the specified compositions andbeing bestowed with plastic strains of 1-5% are useful as fire-proofmembers due to their excellent ductility at a room temperature as wellas high yield strength at 600° C. compared with comparative steelstrips. TABLE 7 ALLOYING COMPOSITIONS OF STEELS USED IN EXAMPLE 4 STEELCOMPONENTS AND CONTENTS    (unit: wt. %) MARK C Si Mn P S Al Cu Ni Cr MoTi Nb V W NOTE XXVIII 0.09 0.01 0.51 0.014 0.004 0.029 0.01 0.01 0.03 —0.050 0.040 — — COMPARATIVE EXAMPLE XXIX 0.07 0.41 0.52 0.012 0.0080.031 0.55 0.46 0.40 0.35 — — — — PRESENT INVENTION XXX 0.08 0.03 0.770.011 0.006 0.038 0.02 0.02 0.02 0.35 — — 0.065 0.010 XXXI 0.11 0.251.08 0.008 0.010 0.039 0.02 0.01 0.01 0.45 — 0.025 — —

74. TABLE 8 EFFECTS OF COLD-ROLLING REDUCTION RATIO ON TENSILEPROPERTIES OF COLD-ROLLED STEEL STRIPS AND HOT-DIP COATED STEEL STRIPSAT A ROOM TEMPERATURE AND 600° C. TENSILE PROPERTIES AT A ROOM THICKNESSTEMPERATURE YIELD REDUCTION OF TEST TENSILE YIELD ELONGA- STRENGTHEXAMPLE STEEL RATIO* PIECE STRENGTH STRENGTH TION (MPa) No. MARKPROCESS* (%) (mm) (MPa) (MPa) (%) AT 600° C. NOTE 36 XXVIII 2 0 1.0 595510 28 121 COMPARATIVE EXAMPLE 37 XXVIII 2 3 1.0 601 562 25 140COMPARATIVE EXAMPLE 38 XXIX 1 3 0.8 462 420 29 200 PRESENT INVENTION 39XXIX 1 6 0.8 478 454 14 227 COMPARATIVE EXAMPLE 40 XXX 1 2.5 0.8 468 42927 214 PRESENT INVENTION 41 XXX 1 8 0.8 487 465 13 222 COMPARATIVEEXAMPLE 42 XXX 2 1.5 0.8 467 438 27 208 PRESENT INVENTION 43 XXX 2 3.50.8 477 450 26 212 PRESENT INVENTION 44 XXX 3 2 0.8 461 431 25 207PRESENT INVENTION 45 XXXI 1 2 0.6 552 513 24 238 PRESENT INVENTION

75. A cold-rolled steel strip or a hot-dip coated cold-rolled steelstrip according to the present invention is excellent in formability andhigh-temperature properties necessary for use as a fire-proof buildingmember. Since there is not required any special changes in manufacturingprocess from a steel making step to an annealing or hot-dip coatingstep, the steel strip is advantageously manufactured in an industrialpoint of view. A fire-proof property of the steel strip is furtherimproved by cold-rolling the steel strip with such a slight duty toinduce a plastic strain of 1-5% after annealing or hot-dip coating.

What is claimed is;
 1. A cold-rolled steel strip for use as a fire-proofbuilding member consisting of 0.01-0.25 wt. % C, up to 1.5 wt. % Si,0.05-2.5 wt. % Mn, up to 0.1 wt. % P, no more than 0.02 wt. % S,0.005-0.1 wt. % Al, 0.05-1.0 wt. % Mo and the balance being essentiallyFe except inevitable impurities.
 2. The cold-rolled steel strip definedin claim 1 , which further contains 0.005-0.2 wt. % one or more selectedfrom Ti, Nb, V and W.
 3. The cold-rolled steel strip defined in claim 1, which further contains one or more of 0.05-0.6 wt. % Cu, 0.05-0.6 wt.% Ni, 0.05-3.0 wt. % Cr and 0.0003-0.003 wt. % B.
 4. The cold-rolledsteel strip defined in claim 1 , which further contains 0.005-0.2 wt. %one or more selected from Ti, Nb, V and W, and one or more of 0.05-0.6wt. % Cu, 0.05-0.6 wt. % Ni, 0.05-3.0 wt. % Cr and 0.0003-0.003 wt. % B.5. A hot-dip coated cold-rolled steel strip for use as a fire-proofbuilding member consisting of 0.01-0.25 wt. % C, up to 1.5 wt. % Si,0.05-2.5 wt. % Mn, up to 0.1 wt. % P, no more than 0.02 wt. % S,0.005-0.1 wt. % Al, 0.05-1.0 wt. % Mo and the balance being essentiallyFe except inevitable impurities.
 6. The hot-dip coated cold-rolled steelstrip defined in claim 5 , which further contains 0.005-0.2 wt. % one ormore selected from Ti, Nb, V and W.
 7. The hot-dip coated cold-rolledsteel strip defined in claim 5 , which further contains one or more of0.05-0.6 wt. % Cu, 0.05-0.6 wt. % Ni, 0.05-3.0 wt. % Cr and 0.0003-0.003wt. % B.
 8. The hot-dip coated cold-rolled steel strip defined in claim5 , which further contains 0.005-0.2 wt. % one or more selected from Ti,Nb, V and W, and one or more of 0.05-0.6 wt. % Cu, 0.05-0.6 wt. % Ni,0.05-3.0 wt. % Or and 0.0003-0.003 wt. % B.
 9. A method of manufacturinga cold-rolled steel strip for use as a fire-proof building member,comprising the steps of: preparing a slab having composition consistingof 0.01-0.25 wt. % C, up to 1.5 wt. % Si, 0.05-2.5 wt. % Mn, up to 0.1wt. % P, no more than 0.02 wt. % S, 0.005-0.1 wt. % Al, 0.05-1.0 wt. %Mo, optionally 0.005-0.2 wt. % one or more selected from Ti, Nb, V andW, optionally one or more of 0.05-0.6 wt. % Cu, 0.05-0.6 wt. % Ni,0.05-3.0 wt. % Cr and 0.0003-0.003 wt. % B and the balance beingessentially Fe except inevitable impurities; heating said slab at1000-1250° C.; hot-rolling the heated slab at a finishing temperature of800-950° C.; coiling the hot-rolled steel strip at 400-700° C.; acidpickling the hot-rolled steel strip; cold-rolling the pickled steelstrip at a reduction ratio of 40-90%; annealing the cold-rolled steelstrip at a temperature above its recrystallization temperature but below950° C.; and optionally cold-rolling the annealed steel strip with sucha slight duty to induce a plastic strain of 1-5% to said steel strip.10. The method defined in claim 9 , wherein the cold-rolled steel stripis box-annealed or continuously annealed.
 11. A method of manufacturinga hot-dip coated cold-rolled steel strip for use as a fire-proofbuilding member, comprising the steps of: preparing a slab havingcomposition consisting of 0.01-0.25 wt. % C, up to 1.5 wt. % Si,0.05-2.5 wt. % Mn, up to 0.1 wt. % P, no more than 0.02 wt. % S,0.005-0.1 wt. % Al, 0.05-1.0 wt. % Mo, optionally 0.005-0.2 wt. % one ormore selected from Ti, Nb, V and W, optionally one or more of 0.05-0.6wt. % Cu, 0.05-0.6 wt. % Ni, 0.05-3.0 wt. % Cr and 0.0003-0.003 wt. % Band the balance being essentially Fe except inevitable impurities;heating said slab at 1000-1250° C.; hot-rolling the heated slab at afinishing temperature of 800-950° C.; coiling the hot-rolled steel stripat 400-700° C.; acid pickling the hot-rolled steel strip; cold-rollingthe pickled steel strip at a reduction ratio of 40-90%; in-line heatingthe cold-rolled steel strip at a temperature above its recrystallizationtemperature but below 950° C. in a continuous hot-dip coatinginstallation; immersing the in-line heated steel strip into a hot-dipcoating bath; and optionally cold-rolling the hot-dip coated steel stripwith such a slight duty to induce a plastic strain of 1-5% to said steelstrip.